Enemy is present, one flier periodically observed (7)
I believe the answer is:
hostile
'enemy' is the definition.
(similar in meaning)
'present one flier periodically observed' is the wordplay.
I cannot really see how this works, but
'present' could be 'host' (I've seen this before) and 'host' is located in the answer.
'one' could be 'i' (Roman numeral) and 'i' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
'is' is the link.
Can you help me to learn more?
(Other definitions for hostile that I've seen before include "Inimical" , "Antagonistic" , "Aggressive" , "Enemy" , "The soil strangely is unfriendly" .)